Rotary scale

ABSTRACT

A rotary ring for use in scale reading apparatus, comprises a flexible ring ( 10 ) which has scale markings ( 14 ) provided on a surface thereof. The rotary ring is mounted onto a machine part ( 22 ) by providing the machine part with a region of increased diameter ( 26 ) and stretching or shrinking the flexible rotary scale ( 10 ) over said region of increased diameter. The manufacturing tolerance of the rotary scale or differential e.g. thermal expansion is thus taken up by stretching or shrinking it into place. The region of increased diameter may comprise an annular protrusion or an O-ring, for example.

The present invention relates to a rotary scale for use in metrologicalscale reading apparatus.

A known form of metrological scale reading apparatus for measuringrelative displacement of two members comprises a scale on one of themembers, having scale marks defining a pattern and a readhead on theother of the members. In an opto-electronic scale reading apparatus, thereadhead includes a light source for illuminating the scale, periodicdiffraction means for interacting with light reflected from the scalemarks to produce interference fringes at the readhead. Relative movementof the scale and readhead cause the interference fringes to moverelative to the readhead. Detecting means are responsive to the movementof the fringes and produce a measure of displacement. Another type ofmetrological scale reading apparatus comprises magnetic scale readingapparatus in which the scale comprises a magnetic grating and whereinpulse signals for each unit displacement of the readhead are used todetermine a measurement value.

An example of such apparatus is disclosed in EP-A-0 207 121 and alsoU.S. Pat. No. 4,974,962, each of which shows the means for illuminatingthe scale and the periodic diffraction means in the read head. U.S. Pat.No. 4,926,566 discloses a method of producing a scale, in the form of aflexible tape produced by rolling. This method may produce a scale inwhich the pitch of the scale marks is, for example, 20 μm or 40 μm.

For measuring rotary displacement, a scale is held around a cylindricalsurface which rotates in use with a shaft or other rotary part relativeto a readhead. This apparatus is typically called a rotary encoder.

Our European Patent application 1094302 discloses a rotary encoder inwhich a ring is used to hold a length of scale in the form of metallictape on an outer surface. The ring is fitted onto a rotary part of amachine. The inner circumferential surface of the ring and the rotarypart of the machine onto which the ring is fitted are both tapered. Thisremoves the need for close tolerances on the diameters of the ring andthe rotary part. In addition it enables the adjustment of the centre ofthe ring.

According to a first aspect of the invention there is provided a rotaryring for use in scale reading apparatus, comprising:

-   -   a flexible ring, the flexible ring having scale markings        provided on a surface thereof.

A second aspect of the present invention provides a system for mountinga rotary ring for use in scale reading apparatus onto a machine part,comprising the rotary ring of claim 1 and co-operating means on saidmachine part, said co-operating means comprising a region of increaseddiameter.

A third aspect of the present invention comprises a method of mounting aflexible rotary scale onto a part of a machine, said part having aregion of increased diameter, the method comprising:

-   -   stretching or shrinking the flexible rotary scale over said        region of increased diameter.

The manufacturing tolerance of the rotary scale or differential e.g.thermal expansion is thus taken up by stretching or shrinking it intoplace.

The region of increased diameter may be integral with the machine part,eg an annular protrusion, or may comprise a separate part, egring-shaped flexible member, such as an O-ring.

The region of increased diameter may comprise a tapered surface. Inaddition, or alternatively, the flexible rotary ring may be providedwith a tapered surface. The tapered surface of the region of increaseddiameter and/or the rotary ring form a self locking taper.

The region of increased diameter may be shaped so that once the flexiblerotary ring is fitted over said region of increased diameter, thecentral region of said rotary ring is substantially parallel with theaxis of said machine part.

The present invention will now be described, by way of example, withreference to the accompanying drawing, in which:

FIG. 1 is a perspective view of the ring of the present invention;

FIG. 2 is a plan view of a length of flexible scale use to make the ringof the present invention;

FIG. 3 is a cross-section of a length of tapered flexible scale;

FIG. 4 is a cross-section of a ring mounted over an annular protrusionon a rotary part of a machine;

FIGS. 5 and 6 are cross-sections of a ring being mounted over a taperedprotrusion on a rotary part of a machine;

FIG. 7 is a cross-section of a ring mounted over an O-ring mounted on arotary part of a machine;

FIG. 8 is a cross-section of a tapered ring mounted on a tapered rotarypart of a machine;

FIG. 9 is a cross-section of a ring mounted on a tapered rotary part ofa machine;

FIG. 10 is a plan view of a garter spring mounted between a flexiblering and machine shaft;

FIG. 11 is a plan view of a corrugated spring mounted between a flexiblering and a machine shaft; and

FIG. 12 is a perspective view of part of the corrugated springillustrated in FIG. 11.

FIG. 1 illustrates a rotary ring of the present invention. A flexiblering 10 is provided with scale marks 14 defining a pattern along onesurface. The scale marks may for example define a periodic pattern toform an incremental scale or define a pseudorandom sequence or discretecodewords to form an absolute position scale. The scale marks may beprovided directly on a surface of the rotary ring or may be provided ona tape which is secured to a surface of the rotary ring. The ring 10 maybe made from a length of linear scale formed from a length of flexibletape 12 as shown in FIG. 2. Such a linear scale is disclosed in ourearlier patent U.S. Pat. No. 4,926,566 and may be made by subjecting alength of tape to an embossing process to form a profile comprising asequence of troughs and crests. The embossing process may entail, forexample, passing the scale between rollers, one of which has a profiledsurface corresponding to the desired profile of the scale.

The rotary ring 10 is formed by bending the linear scale 12 into a ringand joining the two opposite ends 16,18 together by any suitable method,for example by welding. In particular, the ends of the tape may be buttwelded or laser welded together.

Alternatively the scale marks may be put onto a flexible ring by anysuitable technique, for example etching. The flexible ring may be formedby deposition or machining from a solid, for example.

FIG. 1 shows the ring 10 having the scale markings 14 on the outercircumferential surface. Alternatively the scale markings may beprovided on the inner circumferential surface. In this case mounting thering onto a machine part is inverted such that the outer circumferentialsurface of the ring is mounted on the machine part.

The ring 10 shown in FIG. 1 may be provided with a tapered surface 20,as shown by the cross-section of the ring in FIG. 3.

This flexible ring has the advantage that it is smaller and lighter thansolid rings.

A further advantage is that the flexible ring can fit within the desiredapplication with the minimum of space or constraint on design. Use ofthe flexible ring has almost the same effect as putting the graduationsdirectly on the prime components but with the advantage that theflexible ring may be replaced if damaged.

The ring is mounted on a rotary part of a machine and a readhead ismounted on a stationary part of the machine. Alternatively, the ring maybe mounted on a stationary part of the machine and the readhead may bemounted on a rotary part of the machine. In previous types of rotaryrings, mounting screws may be provided to fix the ring to the rotarypart. This is not possible with the thin flexible ring and thus othermeans of fixing the ring onto the rotary part must be used. When thering is mounted on the rotary part of the machine, it is desirable thatthe manufacturing tolerance of the ring is taken up.

Methods of mounting the flexible ring onto a rotary part of a machinewill now be described with reference to FIGS. 4-9.

FIGS. 4-7 all illustrate the ring 10 fitted onto a rotary part 22 of amachine in which the rotary part has been provided with features tosecure the ring in place.

FIG. 4 illustrates a rotary part 22 with an annular protrusion 26. Thering 10 is pushed onto this protrusion 26 such that the protrusion islocated at the centre of the scale, causing the scale to deflect aroundthe protrusion, fixing it in place. Although the scale is deflected whenin position, the readhead 24 is positioned to read the centre of thescale which is negligibly distorted.

FIGS. 5 and 6 illustrates a different feature on the rotary part of themachine. In this embodiment the rotary part 22 is provided with atapered surface 28 at the upper section of the rotary part 22 and arecessed portion 30 at the lower section of the rotary part. As shown inFIG. 5, the ring 10 is pushed onto the tapered surface 28, whichstretches the ring, taking up the manufacturing tolerance. Once thebottom portion of the ring reaches the recess 30, the ring will be heldin position, as shown in FIG. 6, deflected over the tapered surface 28,with one side deflected on the tapered surface 28, the other sidedeflected into the recess 30 and the central portion at the widest partof the taper. At the widest part of the scale, the taper may flattenout, to provide a flatter area for the centre of the ring to minimisedistortions of the scale as read by the readhead.

The features of the rotary part are not necessarily integral with therotary part. FIG. 7 illustrates an embodiment in which the feature isprovided by an O-ring 32 fitted around the rotary part 22. Thecircumferential surface of the rotary part may be provided with aannular recess 34 in which to locate the O-ring 32. The ring 10 ispushed over the O-ring 32 in the same manner as in the otherembodiments.

The “o”-ring may include any ring-shaped flexible arrangement, forexample a garter spring 40 as illustrated in FIG. 10 or a spring 42having a corrugated cross-section as illustrated in FIG. 11 and shown inmore detail in FIG. 12.

FIGS. 8 and 9 illustrate a rotary part 22 of a machine provided with atapered surface 36. In FIG. 8 the ring 10 also has a tapered surface 38,the tapered surface corresponding to that on the tapered rotary part.The tapered ring is pushed down over the tapered rotary part and as thediameter of the rotary part increases down the taper, the manufacturingtolerance of the ring is taken up. The taper angles of the taperedrotary part and ring are chosen such that the taper is a self-lockingtaper. In a locking taper, the angle of friction is less than thecoefficient of friction and the frictional forces between the parts(i.e. the ring and rotary part) are so great that they will not rotateor move with respect to one another. Thus no other clamping forces arerequired to keep the parts joined together and a considerable force isrequired to separate the two parts.

In FIG. 9 a non-tapered ring 10 is pushed over the tapered rotary part22. As the ring 10 is pushed over the tapered surface 36 of the rotarypart 22, the ring 22 is stretched. As with the tapered ring, this ringis held in place by friction.

The ring of the present invention may also be fitted onto the rotarypart by shrink fitting or use of adhesive.

Shrink fitting is suitable, for example, for any of the mountingtechniques described with reference to FIGS. 4-9 or onto a machine shaftwith parallel sides.

The above methods have the advantage that the ring may be quickly andeasily fitted into place on the rotary ring without the need formounting fixtures, such as screws. The process of stretching orshrinking the flexible scale into places ensures that the manufacturingtolerances of the scale are taken up.

If required, the ring may be adjusted axially to provide a fine radialrun out correction, for example as disclosed in European PatentApplication No. EP1094302.

This invention is not limited to use with optical scales. It is alsosuitable, for example, for use with magnetic scales.

1. A rotary ring for use in scale reading apparatus, comprising: a flexible ring, the flexible ring having scale markings provided on a surface thereof.
 2. A system for mounting a rotary ring for use in scale reading apparatus onto a machine part, comprising the rotary ring of claim 1 and co-operating means on one or both of said machine part and said rotary ring, said co-operating means comprising a region of increased diameter. 3-19. (canceled)
 20. A system according to claim 2 wherein the cooperating means is located on the machine part.
 21. A system according to claim 20 wherein the region of increased diameter is integral with the machine part.
 22. A system according to claim 20 wherein the region of increased diameter is not integral with the machine part.
 23. A system according to claim 2 wherein the region of increased diameter comprises an annular protrusion.
 24. A system according to claim 2 wherein the region of increased diameter comprises a tapered surface.
 25. A system according to claim 2 wherein the flexible rotary ring is provided with a tapered surface.
 26. A system according to claim 2 wherein at least one of the region of increased diameter and the rotary ring is provided with a tapered surface and form a self locking taper.
 27. A system according to claim 22 wherein the region of increased diameter comprises a ring-shaped flexible member.
 28. A system according to claim 2 wherein the region of increased diameter is shaped so that once the flexible rotary ring is fitted over said region of increased diameter, the central region of said rotary ring is substantially parallel with the axis of said machine part.
 29. A method of mounting a flexible rotary scale onto a part of a machine, the method comprising: stretching or shrinking the flexible rotary scale onto the part.
 30. A method of mounting a flexible rotary scale onto a part of the machine according to claim 29, wherein the part has a region of increased diameter and the method includes
 31. A method of mounting a flexible rotary scale onto a part of a machine according to claim 29 wherein the region of increased diameter is integral with the part of the machine.
 32. A method of mounting a flexible rotary scale onto a part of a machine according to claim 29 wherein the region of increased diameter is not integral with the part of the machine.
 33. A method of mounting a flexible rotary scale onto a part of a machine according to claim 29 wherein the region of increased diameter comprises an annular protrusion.
 34. A method of mounting a flexible rotary scale onto a part of a machine according to claim 29 wherein the region of increased diameters comprises a tapered surface.
 35. A method of mounting a flexible rotary scale onto a part of a machine according to claim 29 wherein the flexible rotary scale is provided with a tapered surface.
 36. A method of mounting a flexible rotary scale onto a part of a machine according to claim 29 wherein at least one of the region of increased diameter and the flexible rotary scale are provided with a tapered surface and form a self locking taper.
 37. A method of mounting a flexible rotary scale onto a part of a machine according to claim 32 wherein the region of increased diameter comprises a ring-shaped member.
 38. A method of mounting a flexible rotary scale onto a part of a machine according to claim 29 wherein the region of increased diameter is shaped so that once the flexible rotary scale is fitted over said region of increased diameter, the central region of said flexible rotary scale is substantially parallel with the axis of said part. 